Such combinations are generally known in the field of water heating kettles, flow heaters and heaters for food, like baking plates used in fast food restaurants or braising pans. In these prior art combinations of heaters and dissipaters, the heating element is attached to the heat dissipater by bolts and nuts, external clamping means or a bonding layer between the heating element and the heat dissipater.
The proprietor's granted patent GB-A-2 351 894 discloses a brazed connection between a heater and a heat dispersion plate. However, in this prior art solution the surface area of the heating element is substantially smaller than that of the heat dissipater. Hence an additional heat dispersion layer is present between the heating element substrate and the heat dissipater. The size of the heat dispersion layer is larger than that of the heating element, so that the heat transfer takes place over only a part of the surface of the dispersion layer which will result in a slower heat up time and a temperature gradient across the heat dissipater which may be a problem, for example in flow through heater applications.
Prior art flow through heaters that rely upon magnesium filled heating elements are also well known and generally fall into two categories.
The first category relies upon a spiral tube and a magnesium-filled sheathed heating element being diecast or stamped into an alloy or aluminium casting which acts as a dissipater. Generally the power to mass ratio is very low, typically between 1.5 and 2 watts per gram. These heater assemblies are suitable to heat water close to boiling point providing the flow rate of the water to be heated is constant. However these heater types are slow to heat up and slow to react to changing conditions such as the flow rate of the water. Additional tubular heaters may be incorporated, but these add to the cost and increase the size and mass of the assembly.
The second type relies upon one or more magnesium filled sheathed heating elements being attached to a straight tube. This type does have higher power to mass ratio however the heat transfer is slow, which is satisfactory if run at lower temperatures, for example in a washing machine; however they tend to overheat and cause steam if temperatures closer to the boiling point are required.
Prior art flow through heaters are also known that incorporate thick film heaters as a heat source, for example, as described in the proprietor's patent publication WO-A-2005/080885. However, these also suffer from problems related to the control of the water temperature and complexity of assembly.
Furthermore, prior art flow through heaters are also known to have channels provided in the flow through heater assembly, for example as described in the proprietor's patent publication WO-A-2007/037694. However, these also suffer from problems relating to complexity of assembly due to the increased number of components used to form the flow through heater assembly, and therefore add cost and increase manufacturing time of the assembly.
In view of the recent trend to smaller and quicker reacting liquid heating devices that do not rely upon the need to store heated liquid, it is advantageous to increase the power density of the heating means without the risk of the temperature overshooting to boiling point. When increasing the power density it is essential to ensure the integrity of the mechanical and thermal contact between the heating element and the heat dissipater. With prior art techniques in connecting the heating element with the heat dissipater, lowering the thermal mass may lead to warping of one or both of the components, which can result in a reduced contact area between the heating element and the dissipater reducing the optimal heat transfer.
This problem is in particular present in situations where in the overall power density is high, for instance higher that 8 W/cm2. It would be advantageous to provide a high power density combination wherein the transfer of heat from the heater to the heat dissipater is maintained, throughout the lifetime of the component.